Electromagnetic machines, such as pumps intended to circulate a conducting liquid material, devices for braking the circulation of a conducting liquid in a duct, linear motors or alternatively magnetohydrodynamic (MHD) generators, which include a straight tubular duct intended to receive a conducting material which can circulate in the duct, and an inductor arranged coaxially around the duct, are known.
The inductor generally includes a magnetic circuit formed by magnetic sheet metal combs forming annular notches coaxial with the duct, in each of which notches an annular winding, coaxial with the duct, is arranged.
The notches of the combs of the inductor of the electromagnetic machines of known type, in which the windings are arranged, are separated by parts, called teeth, which project radially inward.
The windings, which are distributed along the axial direction of the electromagnetic machine, are each connected to one phase of a polyphase current source, so that the inductor creates a field sliding in the axial direction, along the tubular duct.
Induced currents are thus created in the conducting material inside the duct, so that the material is circulated in the axial sense, by the combined effect of the induced currents and the sliding magnetic field of the inductor. In order to facilitate closure of the field lines inside the tubular duct, in particular in the case when the electromagnetic machine is used as a pump, it is proposed to place a magnetic core in a concentric arrangement inside the duct.
Such a pump can be used in particular for circulating a liquid metal, such as sodium, used as a heat-exchange fluid in fast-neutron nuclear reactors.
In such pumps, and more generally in the case of electromagnetic machines, whatever their use, it is desired to increase the efficiency of the machine, i.e., for example, in the case of a pump, the ratio of the hydraulic power to the electrical power consumed by the pump.
In the case of linear-induction electromagnetic pumps which are known and used, this efficiency is always substantially less than 50%.
Electromagnetic machines are also known, such as pumps including a duct of flat shape, for example with a rectangular cross-section, and a flat inductor including straight and mutually parallel notches which are separated by straight teeth. The inductor is placed parallel and facing one of the large plane faces of the duct, so that the teeth point towards the duct.
In the linear-induction electromagnetic machines of known the, the teeth of the inductor are all the same length and are regularly distributed in the axial direction.
The distance between the successive windings of the inductor is therefore constant.
Furthermore, the successive windings of the inductor are generally identical and all produce the same number of amps/turn.
As indicated above, the efficiency of such an electromagnetic machine used as a pump is substantially less than 50%, and generally close to 40%.
In order to increase the efficiency of linear-induction electromagnetic pumps, such as pumps used for circulating liquid sodium in fast-neutron nuclear reactors, various solutions are proposed which make it possible to vary either the speed of circulation of the fluid in the axial direction, or alternatively the distribution of the magnetic flux.
It has for example been proposed to use a duct having conical parts so as to vary the passage cross-section and therefore the speed of the fluid in the axial direction of the inductor corresponding to the direction of circulation of the fluid.
Producing such conical parts on the fluid circulation duct raises difficulties in design and manufacture of the duct.
It has also been proposed to produce a linear-induction electromagnetic pump whose inductor has an additional pole at one of its ends, consisting of a longer additional tooth, or of an extension of the last tooth of an inductor of standard type.
Although such a pump makes it possible to achieve an efficiency of close to 50%, this solution cannot be adopted generally, insofar as it has the drawback of increasing the bulk and the mass of the pump. In the case of pumps intended for conveying the liquid sodium of fast-neutron nuclear reactors with a very high hourly throughput, for example of the order of 600 m.sup.3 /hour or even 11,000 m.sup.3 /hour, the axial size of the pump reaches a value of several meters, and it is generally undesirable to increase this size further.
It has also been proposed to place two windings of different pole pitches on top of each other in each of the notches of the combs of the inductor. This solution leads to a high investment cost which is due to the complexity of the electrical supply or of actually producing the winding itself.
In GB-A-2,064,229, it was proposed, with the aim of decreasing the reactive power of an electromagnetic machine, to use a stator including two successive parts, the first having regularly spaced pole pitches, and the second having regularly spaced pole pitches, but with shorter pitches than the first. Such an arrangement does not substantially improve the efficiency of the electromagnetic machine.
Linear-induction electromagnetic machine having both high efficiency and moderate size and cost, in particular those which could operate as a pump with high power and high throughput were known.